The value of soil testing

April 03, 2007|By Bob Fanning Extension Educator Lyman County

By Bob Fanning Extension Educator Lyman County Soil testing has long been a recommended practice in crop production. It makes sense to know the levels of various soil nutrients before making decisions about how much fertilizer to apply. With fertilizer prices high, and knowing that Nitrogen carryover levels tend to be higher than average following a drought, soil testing becomes more important than ever. A producer recently received his soil test results from the SDSU Soil Testing Lab and they indicated he could save a lot of money by following the rate recommendations. There were 14 fields involved, representing just over 1,000 acres, with a variety of previous and intended crops. One could argue that a few of the producers yield goals for the intended crops may have been a little optimistic, but realistic adjustments would have a minor effect on recommended fertilizer rates. Most, but not all of the nitrogen soil test levels were fairly high, which is expected following a drought year when yields are lower than average. Using the soil test results and fertilizing according to his yield goals and SDSU recommendations, he would apply about 69 tons of urea at a cost of about $25,000 if the cost was $0.40 per lb of Nitrogen. Had the producer assumed each field carried over 40 lbs per acre of Nitrogen and fertilized according to SDSU recommendations, he would apply about 127 tons of urea at a cost of almost $47,000. That is a savings of nearly $22,000. If he had fertilized assuming 40 lbs per acre of Nitrogen carry over, he only would have under fertilized one field, by 7 lbs per acre. If he had assumed each field carried over 50 lbs per acre of Nitrogen, he would apply just under 115 tons of urea at a cost of over $42,000. Soil testing saved over $17,000 compared to this scenario. The Phosphorus soil tests indicated he had been regularly applying at least the amount of Phosphorus that crops had been removing. For nine of the 14 fields, the level was high enough that no Phosphorus was recommended. By following SDSU recommendations, if the Phosphorus costs $0.25 per lb of P2O5, he will apply just over 4,900 lbs of P2O5 at a cost of about $1,200. If he applied Phosphorus at the rate his crops would expect to remove at his yield goals, he would apply over 32,000 lbs of P2O5 at a cost of over $8,000. Phosphorus fertilizer savings would be nearly $7,000. Combined, his fertilizer savings could be over $28,000 by soil testing. Also, two of the fields had no Nitrogen fertilizer recommendation, so application costs for over 150 acres can be saved. At $11.00 per sample, plus postage, the analysis cost this producer under $160, a nice return for the time and effort of taking soil samples. Of course we know that crop production, soil sampling and testing and fertilizer recommendations are not an exact science. Simply taking soil tests and applying fertilizer according to university recommendations does not guarantee your crop will attain the yield goal you set. Precipitation, temperature, weed pressure, disease, insects and other conditions certainly have a large impact on crop yields. Soil sampling requires time and effort, but can return large dividends. A great deal of research goes into the SDSU fertilizer recommendations, and using them based on proper soil sampling can improve your bottom line.